Analog trigger techniques are widely used in digital oscilloscopes and data acquisition devices. Analog triggers have well known disadvantages. Often an analog trigger creates trigger jitter as a result of inaccurate analog “trigger interpolation” (time-to-digital conversion), the procedure used to determine the distance between a trigger event and a sampling clock of an analog-to-digital converter (ADC). Another disadvantage of an analog trigger is that it is difficult to implement a sophisticated trigger, such as a pattern trigger or a trigger in the frequency domain.
The above-mentioned problems can be solved by use of a digital trigger technique. The challenge for a digital trigger, is that the digital trigger must work in real time.
In U.S. Pat. No. 6,892,150, a four channel digital oscilloscope is disclosed, including an ADC. In the disclosed oscilloscope, an analog trigger is combined with a digital trigger which is generated in real time by processor unit built on a FPGA for high speed processing. The digital data from the ADC is supplied simultaneously to an acquisition memory and to a digital trigger processor. However, in the disclosed oscilloscope, the digital data from the ADC is not equalized. For this reason, the signal is generally distorted and creates mis-triggers. ADC equalization (for example, equalization of frequency response, group delay equalization) is especially important in high sampling rate ADC's, which typically include multiple time-interleaved ADC's necessary for the high sampling rate
In U.S. Pat. No. 7,860,674, a digital triggering system is disclosed for an oscilloscope. In that patent, an equalizer is used after an ADC. The equalized data is distributed to main data channel (for observation on the scope screen) and to a digital trigger processor. As a result, the equalizer works in real time. However, extra filtering is required for the trigger channel, like LPF, HPF, BPF etc., which must be also implemented in real time. The extra resources involved to implement real time equalizer and real time trigger filters are significant and require costs associated with the devices needed to perform the necessary signal processing.